Terrestrial productivity depends strongly on the availability of water in the environment (Lieth 1972). Measuring the availability and movement of water among soil, plants, and the atmosphere requires methods from ecosystem studies, plant physiology, soil science, and biogeochemistry (Casper and Jackson 1997). Techniques for estimating the water status of soils have been reviewed recently (Rundel and Jarrell 1989; Boyer 1995) and those for measuring plant water status at cellular and whole-plant levels are thoroughly described in physiological ecology and plant physiology texts (e.g., Koide et al. 1989; Kramer and Boyer 1995). Rather than reviewing all methods for estimating plant and soil water, we emphasize techniques that are increasing in importance or changing rapidly for ecosystem studies. We include methods for determining the water content of soil, the availability of that water for plant uptake, and the transport of water through the plant (Fig. 1). Since many of these methods are also used in physiological ecology, our chapter is designed to bridge the gap between the scales of physiological and ecosystem ecology, as a foundation for other contributions in this book.
We begin by defining terms that describe water in the environment including a brief discussion of water potential, the currency that allows the water status of soil, plants, and the atmosphere to be compared and the direction of flow to be predicted (Slatyer and Taylor 1960). We describe various techniques for measuring soil water in the field, emphasizing such recent innovations as time-domain reflectometry and remotely sensed data. We also discuss methods for estimating the vegetative component of ecosystem water fluxes, including sap-flow measurement and whole root/shoot hydraulic conductivity. Such techniques for estimating whole-plant water use are important for interpreting canopy and ecosystem water fluxes in eddy covariance and other net ecosystem approaches (see chapters 2.1 and 2.2, this volume). We summarize the advantages and disadvantages of various techniques and recommend some future directions for research.
In (OE Sala, RB Jackson, HA Mooney, RW Howarth, eds.) [